OPC Drums

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Question: What are OPC drums, and why are they the most commonly used photoreceptors in office equipment today?

Answer: "OPC" stands for organic photoconductor. The term "organic" indicates that the photoreceptor's coating was manufactured from carbon-based chemical compounds -- specifically, photoconductive polymers synthesized from raw materials, that are obtained by refining fossil fuels such as petroleum. OPC drums are generally considered the most "environmentally friendly" photoreceptors available today -- primarily because their designers and manufacturers consciously utilize nonhazardous raw materials. In fact, all materials must pass strict material safety tests before they can be used in OPC manufacturing. This ensures that OPC drums are, in fact, environmentally friendly alternatives to more hazardous photoreceptors such as arsenic triselenide (As2Se3) and selenium tellurium (SeTe) drums.

OPC Physical Characteristics

The most commonly utilized OPC drums in today's Japanese-designed copiers are manufactured to receive a negative charge. From innermost to outermost layer, they typically consist of an aluminum substrate, undercoat (or "blocking") layer (UCL), charge generation layer (CGL), and charge transport layer (CTL).

The aluminum substrate facilitates photoconductivity physically and electrically, but does not play an active role in the electro-photographic process. Its primary role is to provide structural and mechanical support, as well as an electrical path to ground.

The undercoat layer (UCL) acts as an interface between the substrate and photoconductive layers, to provide adhesion and prevent undesirable charge "leakage" that can adversely affect copy quality. Like the substrate, it does not play an active role in the electrophotographic process, but provides an electrical path to ground. Common UCL materials include aluminum oxide, anodized aluminum, and various resistive polymers.

The charge generation layer (CGL) is extremely thin, typically ranging from only 0.1 to 1.0 micron in thickness. (As a point of reference, the average human hair is 50 microns in diameter!) Its color, which typically determines the apparent color of the OPC drum itself, depends on the specific materials it contains. The light-sensitivity of the CGL is a critical factor in OPC performance, and can be a limiting factor for the copy speed at which an OPC can function effectively.

The charge transport layer (CTL) is the outermost layer of an OPC drum, and is typically about 20 to 30 microns thick. It is essentially transparent, allowing light to pass directly through to the CGL. Just as the CGL primarily determines an OPC's light-sensitivity, the CTL primarily determines its charge acceptance and charge transport rate. As the outermost layer, the CTL is contacted by toner, developer, paper, the drum cleaning blade (or brush), ozone, and other potentially abrasive and/or contaminating agents. Consequently, CTL wear characteristics, such as durability and abrasion-resistance, are critical factors in the potential life of an OPC drum.

While the above description applies to most OPC drums in use today, other types exist. For example, some Mita copiers utilize positive-charge OPC drums with a combined CGL and CTL (referred to as "monolayer" OPC drums). Since this single layer determines all the electrical and physical characteristics of the coating (including charge acceptance, photosensitivity, and wear-resistance) it must be formulated and manufactured with extreme precision. Only Mita and Katun have successfully introduced OPC drums of this type. Positive-charge OPC drums typically have a shorter life than "standard" OPC drums, because their monolayer is less abrasion-resistant when incorporating "softer" materials usually confined to the CGL.

Benefits of OPC Technology

There are many significant reasons for the office equipment industry's relatively rapid, extensive conversion to OPC drums. First, advances in coating materials and technology have made possible the manufacture of more light-sensitive, more durable OPC drums suitable for use in a wide range of applications, including copiers operating at extremely high speeds (e.g., 75 cpm or above). The OPC drums utilized in most of today's newer, Japanese-designed segment 3, 4, and 5 machines provide levels of copy quality and long life formerly attainable only with As2Se3 drums.

Increased environmental concern is also a major factor in the industry's migration to OPC technology. The increased push toward global environmental awareness has led to expanded, intensified restrictions on disposal of cadmium sulfide (CdS) and selenium-based (As2Se3 and SeTe) photoreceptors. OPC drums, which are not classified as hazardous waste, are the most conveniently disposable alternative available (used OPC drums should always be submitted to an aluminum recycler rather than discarded in landfills, however). This consideration also makes OPC drums the logical choice for use in replaceable drum units/cartridges used in a wide range of popular Canon machine applications.

Another reason Japanese OEMs prefer OPC technology is that its manufacturing process is generally much less expensive than the time-consuming, less efficient process required to manufacture selenium-based photoreceptors. Dip coating is the most common method of manufacturing OPC drums; this is a "continuous" manufacturing process, whereas the vapor-deposition process used to manufacture selenium-based and amorphous silicon (a-Si) drums is a "batch" process: it requires sequential placement of batches of drums in vacuum chambers to form the different coating layers. This fundamental difference in manufacturing requirements contributes to the relatively lower costs of OPC drums.

Due to the significant benefits previously mentioned -- as well as the continued use of OPCs in laser printers and other digital applications (including digital copiers) -- abundant application of OPC coating technology will most likely continue. Consequently, OPC research and development will remain a high priority in the office equipment industry well into the future.

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